The invention relates to an adjustable torsion-spring slipping clutch, which in normal operation provides a constant torque drive from a faster rotating driving section to a drive section; and in particular to such a clutch useful for apparatus for recording and/or playing back signals on a magnetic tape, the clutch comprising first and second coaxially arranged clutch sections which are rotatable relative to each other about an axis of rotation, and a coaxially arranged cylindrical helical torsion spring which at one end is connected to the first clutch section, the second clutch section comprising a sleeve which is substantially circular in a cross-section perpendicular to the axis of rotation and around which turns of the spring are wrapped for transmitting torque between the clutch sections by the friction between the spring and the sleeve, which turns are adjustable in diameter for determining the torque to be transmitted between the clutch sections.
A clutch of this type is known from the published Austrian Patent Application A 8026/77, to which U.S. Pat. No. 4,258,556 corresponds. In this known clutch the sleeve of the second clutch section is conical, the spring being axially movable relative to the sleeve so as to enable the diameter of the spring turns to be adjusted. Depending on the apex angle of this cone some friction may occur between the turns of the spring under certain circumstances, which inter-turn friction may give rise to a certain inaccuracy in the operation.